Method and apparatus for powering circuitry with on-chip solar cells within a common substrate

ABSTRACT

A system and method for providing power to a light-powered transponder. In order to create a sufficient voltage differential, two different photovoltaic elements are used. The photovoltaic elements generate voltages of different polarities. Because the photovoltaic elements are used independently to generate voltages with different polarities, the present system can achieve a desired voltage differential despite the inherent difficulties presented by the use of a standard CMOS process.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to electronic radio frequencyidentification tags and specifically to small electronic transpondersthat store and transmit information. In particular, the presentinvention relates to the use of light to generate sufficient power fortransponders.

[0002] Electronic transponders are used in a wide variety ofapplications to store and transmit information. A transponder functionsby receiving a transmission request and, in turn, transmitting aresponse. Typically, this response is an identification signal, oftencomprising a serial number.

[0003] In World War II, transponders were used to identify aircraft. Thetransponder assured the requesting aircraft that the associated aircraftwas a friendly aircraft by transmitting an identification code. Earlyversions of electronic transponders supplied power by way of a batteryor a solenoid. However, batteries and solenoids are relatively large,and therefore severely restrict the ability to reduce the size ofelectronic transponders.

[0004] An antenna external to the transponder broadcasted identificationinformation. This external antenna was necessary to generate a RF signalstrong enough to be received and demodulated by a receiver. An externalantenna, however, further increases the size of the transponder.

[0005] Today, transponders are used for a variety of purposes rangingfrom identification of wildlife to electronic article surveillance(EAS). Typically, transponders utilize a radio frequency identification(RFID) system. These systems operate without visual contact. Forexample, EAS systems typically employ a closed loop of a conductivesubstance that responds to a generated radio frequency (RF) field. Thesetransponders, also called tags due to their ability to “tag” a consumeritem to prevent shoplifting, are deactivated when a product ispurchased. To further this goal, EAS systems may transmit a descriptionof the item to which the tag is affixed.

[0006] Transponders are also beneficial for applications where it ishighly desirable to reduce the size of the transponder to very smalldimensions. For example, electronic transponders aid in the detection ofbiomolecules in samples when performing solid-phase assays. U.S. Pat.Nos. 5,641,634, 5,736,332, 5,981,166, and 6,001,571 respectivelydisclose the use of transponders for detecting biomolecules, determiningthe sequence of nucleic acids, screening chemical compounds, andperforming multiplex assays for nucleic acids, and are hereinspecifically incorporated by reference. For these applications, thetransponder must be significantly reduced in size.

[0007] For use in chemically hostile environments, as those often usedin solid-phase assays, external antennas and power sources utilized inearlier prior art transponders needed to be protected. Therefore, theentire transponder, including the power source and antenna, would beenclosed in a protective material, such as a glass bead. This enclosurefurther added to the size of the transponder.

[0008] As disclosed in U.S. Pat. No. 5,641,634, miniature transponders,also referred to as microtransponders, using photovoltaic cells toprovide power have been developed. Photo-activated transponders enablesmaller dimensions. Further, by providing a monolithic assembly thatincludes an antenna, the transponder disclosed in U.S. Pat. No.5,641,634 further enables a reduction in size.

[0009] These transponders are typically formed on a silicon wafer andprotected by a thin layer of silicon dioxide (SiO₂). SiO₂ has the samechemical properties as glass with respect to chemically hostileenvironments. Therefore, the transponder does not need to be enclosed ina glass encasement. Alternatively, the transponder may be coated with avariety of transparent or semitransparent materials, such as plastic orlatex.

[0010] In many applications, it is desirable to have a small transponderthat outputs identification data. It is further desirable to create apurely passive device that does not depend on the operation ofself-contained batteries. Photo-activated transponders provided anadvantage over the prior art due to their inactivity without lightillumination. A narrowly focused laser light source may enable a singletransponder at a time, even when a large number of transponders arepresent in the assay. Only the illuminated transponder transmits dataand other transponders are inactive. The reduction in the number oftransmitting transponders significantly reduces noise level. If thelight source is more broadly applied, an increased number oftransponders may respond. Thus, the light source can be adjusted tocontrol which transponders will respond during an assay.

SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENT

[0011] The present invention is defined by the following claims, andnothing in this section should be taken as a limitation on those claims.By way of introduction, the embodiments described below include a methodand apparatus for supplying power from a light source for a transponder.

[0012] In order to minimize size and cost, it is desirable tomanufacture a transponder using a standard CMOS process on a single die.Standard CMOS processes utilize a common, conductive substrate. Thus,without additional processing, any one photovoltaic element is notcompletely isolated from another.

[0013] In instances where more than one photovoltaic element is used tosupply sufficient power, the common substrate causes difficulties. Forexample, if an increased voltage differential is desired, twophotodiodes may be connected in series. In processes where the twophotodiodes are isolated, the series connection will double the voltageproduced. If the two photodiodes are not isolated, as with the use of asingle die created through a standard CMOS process, an increased voltagepotential may not result.

[0014] In order to supply sufficient power to the logic circuitry of thetransponder, the present invention utilizes two photovoltaic elements.The first photovoltaic element produces a positive voltage. The secondphotovoltaic element produces a negative voltage. Used in conjunction,the voltage difference between the two elements is approximately doublethe voltage potential of any one photovoltaic element. Here, thepositive supply is connected to a load terminal and the negative supplyis connected to an other load terminal. Because the photovoltaicelements are used independently to generate voltages with differentpolarities, the present system can achieve a desired voltagedifferential despite the inherent difficulties presented by the use of astandard CMOS process or a common substrate.

[0015] Additionally, a third photovoltaic element may be used as aseparate power source for the transponder antenna. By using a separatepower supply for the antenna, the logic circuitry is not affected by theoperation of the antenna.

[0016] Further aspects and advantages of the invention are discussedbelow in conjunction with the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic of a prior art system of connecting twoisolated photodiodes in series.

[0018]FIG. 2 is a depiction of an ineffective power supply system foruse with a transponder.

[0019]FIG. 3 is a depiction of an embodiment for providing power fromlight for a transponder.

[0020]FIG. 4 is a depiction of another embodiment for providing powerfrom light for a transponder.

[0021]FIG. 5 is a depiction of a die layout of a transponder utilizingan embodiment for providing power from light.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0022] An embodiment of the present invention uses one photovoltaicelement as a positive power supply and uses another photovoltaic elementas a negative power supply. By using two power supplies with differentpolarities, the embodiment provides an increased voltage differentialfor logic circuits with a common substrate. Such a common substrate maybe formed through the use of standard CMOS process techniques.

[0023] In order to drive the logic circuitry of the transponder, thephotovoltaic elements provide at least 1.0 volts. A single photodiodecan provide 0.7 V.

[0024] The common substrate creates difficulties in designing a powersupply system. FIG. 1 shows two physically isolated photodiodesconnected to produce a 1.4 V power supply. The first photodiode 100 isconnected in series with the second photodiode 105. The photodiodesgenerate a voltage differential through the PN junctions 110 and 115.The cathode of the first photodiode 120 is connected to ground. Theanode 125 of the first photodiode yields +0.7 V. The cathode of thesecond photodiode 130 is connected to the anode 125 of the firstphotodiode. As with the first photodiode 100, the second photodiode 105raises the voltage +0.7 V. Thus, the anode 135 of the second photodiodeyields +1.4 V. Because the photodiodes 100, 105 are physically isolated,the two photodiodes 100,105 may be connected in series to double thevoltage differential. If two PN diodes were arranged in series on acommon substrate, however, the desired voltage differential may notresult because the diode cathodes are not isolated by a dielectric.

[0025] As shown in FIG. 2, the N-wells 205, 215 each share a commonconductive P-type substrate 200 which also acts as a second anode toeach. The common substrate 200 is ground for both the power supply andload circuits. A first N-well 205 is located in the substrate 200.Independent of any of the connections, the first N-well would yieldapproximately −0.7V with respect to ground. Connecting the first N-wellto ground forces the first N-well to 0V. This connection now allows a P+implant 210 in the first N-well 205 to yield a positive voltage. Thisfirst P+ implant 210 in the N-well 205 effectively supplies +0.7 V. Asimilar arrangement connected in series, however, may not produce 1.4 V.Here, the N-well 215 of the second photovoltaic element is connected tothe P+ implant 210 of the first photovoltaic element. Again, independentof any of the connections, the second N-well by itself would yieldapproximately −0.7V with respect to ground. At first glance, it mayappear that connecting the second N-well 215 to the P+ 210 of the firstphotovoltaic element would force the N-well 215 to +0.7V and that a+1.4V potential could then be achieved at the second P+ 200. Forsimilarly sized devices, this does not occur because the second PNjunction of the second photovoltaic element (that of the N-well 215 andthe substrate 200) overpowers the first photovoltaic cell by pullingdown the voltage differential. Thus, the second P+ implant 220 does notyield 1.4 V.

[0026] The second PN junction of the second photovoltaic elementdominates the power supply and yields an overall negative voltage at theconnection between the first P+ 210 and the second N-well 215. As aresult, the voltage at the second P+ 210 is approximately 0V. Thisoccurs because an N-well P substrate region has a stronger currentproducing capability than a P+ implant-N-well region.

[0027] The inability to effectively isolate the two power supplies in astandard CMOS process precludes the effective use of two photovoltaicpower supplies in series. Thus, the use of two positive power suppliesor two negative power supplies in order to achieve a higher voltagedifferential is not effective without additional cost or size demands.

[0028] Instead of providing one power terminal to the load and anotherterminal that simply connects the power supply system and the load toground, the present embodiments utilize two power terminals in aconnection with a load. Specifically, the load connects with a positivevoltage terminal and a negative voltage terminal. Because the voltagedifferential between the positive voltage terminal and the negativevoltage terminal is at least one volt, the logic circuitry of thetransponder may be effectively driven.

[0029]FIG. 3 is a graphical depiction of an embodiment of the presentinvention. A silicon die 300 contains a negative supply 305, a positivesupply 310, and a load 315. The negative supply 305 is created throughthe use of an N-well 320 in a P substrate 325. Alternatively, thenegative supply could be created with N+ 330 directly in the P-sub 325,i.e., without the N-well 320. The negative supply 305 generates −0.7 Vthrough a PN junction. The negative supply 305 has an N+ implant 330connected with one terminal connection of the load 315. The negativesupply 305 can supply ample current to the load.

[0030] The positive supply 310 comprises a P+ implant 335 inside anN-well 340 located in the P substrate 325. The positive supply can beviewed as vertical PNP bipolar junction transistor where the substrate325 is the collector, the N-well 340 is the base, and the P+ implant 335within the N-well 340 is the emitter. The positive supply 310 has twojunctions in which a voltage potential may be created. The firstjunction is between the P+ implant 335 and the N-well 340. Here, a +0.7V voltage difference is created. The second junction is between theN-well 340 and the P substrate 325 and a 0.7 V voltage potential may becreated. Simultaneous illumination essentially short circuits thesupply. Without any other connection, illuminating the device wouldresult in 0 V because the junctions would cancel each other out.

[0031] To create a +0.7 V result, the N-well 340 via N+ implant 345 andthe substrate 325 via P+ implant 350 are ground. Grounding the N-well340 prevents the second junction from driving the positive supply 310negatively toward 0V. By tying the N-well 340 (base) to the P substrate325 (collector), which is at 0V, the P+ implant 335 (emitter) within theN-well rises to +0.7 V.

[0032] In this embodiment, two separate power supplies are created witha common substrate 325. The negative power supply 305 comprising anN-well 320 in the P substrate 325 yields approximately −0.7 V. Thepositive power supply 310 comprising a P+ implant 335 in an N-well 340in the P substrate 325 yields approximately +0.7 V. Used in conjunctionthe two power supplies create a voltage differential of approximately1.4 V. This voltage differential is sufficient to drive the circuitlogic represented as the load 315. Other voltages with the same ordifferent amplitude for the positive and negative supplies may be used.

[0033] The embodiment of FIG. 3 uses a P substrate. Thus, the negativesupply 305 uses an N-well and the positive supply 310 uses a P+ implantin an N-well. In the alternative, an N substrate may be used. If an Nsubstrate is used, the negative power supply 305 is represented as anNPN transistor while the positive power supply 310 is represented as aPN diode. The negative power supply 305 comprises an N+ implant inside aP well located in the N substrate. The positive supply 310 comprises ofa P well located in the N substrate.

[0034] Using either embodiment, the power supply source represented as aPN diode (i.e. the negative power supply in the P substrate embodimentand the positive power supply in the N substrate embodiment) providesmore current than the other power supply. The less robust power supply(i.e., the positive power supply in a P substrate embodiment and thenegative power supply in an N substrate embodiment) is examined toensure that sufficient power is provided to the load 315.

[0035] To ensure that sufficient power is delivered by both powersupplies, the size of the less robust power supply is increased. Forexample, as seen in FIG. 5, the cross-sectional area of the negativepower supply 505 may be twice that of a positive power supply 510 in anN substrate embodiment. The increased size will boost the currentsupplying capabilities of the negative power supply (or positive powersupply in a P substrate embodiment).

[0036] As seen in FIG. 4, another embodiment utilizes three powersupplies. An additional power supply 400 is included with a negativepower supply 305 and a positive power supply 310. This additional powersupply 400 may be used to generate power for an antenna used to transmitan output signal. By incorporating a third power supply, the antennadoes not drain power from the logic power supply. As shown in FIG. 4,the additional power supply 400 comprises an N-well 405 in a P substrate325. Through an N+ implant 410, a negative voltage may be delivered tothe antenna. In this regard, the additional power supply 400 isstructurally similar to the negative power supply 305. In thealternative, the additional power supply may be structurally similar tothe positive power supply 310. Further, the additional power supply mayuse a structure different from both the positive and negative powersupply. For example, an N+ to Psub junction without an N-well for theP-sub embodiment may be utilized. Further, the additional power supplymay be used with an N substrate embodiment as either a P+ directly tothe N-sub or a P-well to the N-sub.

[0037]FIG. 5 shows a layout of a transponder using an embodiment of theinvention. The transponder is encompassed on a single die 500. Apositive power supply 505 is located adjacent to a negative power supply510. The positive power supply 505 is larger than the negative powersupply 510. Used in conjunction, the positive power supply 505 and thenegative power supply 510 provide power for logic circuitry 515.

[0038] In an operational transponder, the logic circuitry is shielded toprevent the sources and drains of the NMOS and PMOS devices, as well asthe N-wells of the PMOS devices and P-wells of the NMOS devices, fromoperating as photodiodes. Additionally, to ensure that any light leakagedoes not create voltage potential that could drain the positive powersupply, the logic N-wells are tied to ground. Tying the N-well to groundprevents the PN junction created by the N-well and the P-substrate fromgenerating a negative voltage.

[0039] Additionally, an antenna power supply 520 is provided. In thisembodiment, the antenna power supply 520 comprises an N-well in a Psubstrate. Thus, the antenna 525 is driven by an additional negativepower supply 520.

[0040] A clock recovery circuit 530 and an antenna switch 535 are alsoprovided. The performance of clock recovery is discussed in pending U.S.patent application Ser. No. 09/699,660, filed Oct. 30, 2000, hereinincorporated by reference.

[0041] The antenna 525 comprises a wire loop surrounding the dieutilizing the standard metalization steps in the standard CMOS process.Other antennae, such as microelectomechanical machining (MEMS), may beused. Through the antenna 525, the transponder transmits an outputsignal. The antenna switch 535 controls when the transponder istransmitting.

[0042] It is to be understood that a wide range of changes andmodifications to the embodiments described above will be apparent tothose skilled in the art and are contemplated. It is, therefore,intended that the foregoing detailed description be regarded asillustrative rather than limiting, and that it be understood that it isthe following claims, including all equivalents, that are intended todefine the spirit and scope of the invention.

What is claimed is:
 1. A power supply system for a photo-activatedtransponder, the system comprising: a first photovoltaic power supplyproviding a voltage and a second photovoltaic power supply providing avoltage with a different polarity than the voltage provided by the firstpower supply.
 2. The power supply system in claim 1 wherein said firstphotovoltaic power supply supplies a positive voltage.
 3. The powersupply system in claim 1 wherein said second photovoltaic power supplysupplies a negative voltage.
 4. The power supply system in claim 1wherein said transponder is manufactured using a CMOS process.
 5. Thepower supply system in claim 1 further comprising a substrate connectedwith said first and second power supplies.
 6. The power supply system inclaim 5 wherein said substrate is P-type.
 7. The power supply system inclaim 6 wherein said first photovoltaic power supply comprises a P+implant in an N-well located in said substrate.
 8. The power supplysystem in claim 6 wherein said second photovoltaic power supplycomprises an N-well located in said substrate.
 9. The power supplysystem in claim 6 wherein an N+ contact is located in said substrate.10. The power supply system in claim 5 wherein said substrate is N-type.11. The power supply system in claim 10 wherein said first photovoltaicpower supply comprises an N+ implant in a P-well.
 12. The power supplysystem in claim 10 wherein said first photovoltaic power supplycomprises a P-well located in said substrate.
 13. The power supplysystem in claim 10 wherein a P+ contact is located in said substrate.14. The power supply system in claim 1 wherein said first supply islarger than said second supply.
 15. The power supply system in claim 1further comprising a third power supply.
 16. The power supply system inclaim 15 where said third power supply is operative to supply power toan antenna.
 17. The power supply in claim 5 further comprising a thirdpower supply connected with said substrate.
 18. The power supply inclaim 1 further comprising a load connected with said first and secondphotovoltaic power supplies.
 19. The power supply in claim 18 whereinsaid load comprises digital logic circuitry
 20. The power supply inclaim 18 wherein said load comprises analog logic circuitry.
 21. A powersupply apparatus for a photo-activated transponder, said power supplyapparatus comprising: a P-type substrate; a first N-well connected withsaid P-type substrate; an N+ contact connected with said first N-welloperative to provide power with a negative voltage for a load. a secondN-well connected with said P-type substrate; and a P+ contact connectedwith said second N-well operative to provide power with a positivevoltage for said load.
 22. The power supply apparatus in claim 21further comprising a third n-well connected with said P-type substrateand a second N+ contact connected with said third N-well operative toprovide power to an antenna.
 23. The power supply apparatus in claim 21further comprising a second N+ contact located in said P-type substrateoperative to provide power to an antenna.
 24. A power supply apparatusfor a photo-activated transponder, said power supply apparatuscomprising: an N-type substrate; a first P-well connected with saidN-type substrate; a P+ contact connected with said first P-welloperative to provide power with a positive voltage for a load. a secondP-well connected with said N-type substrate; and an N+ contact connectedwith said second P-well operative to provide power with a negativevoltage for said load.
 25. The power supply apparatus in claim 24further comprising a third P-well connected with said N-type substrateand a second P+ contact connected with said third P-well operative toprovide power to an antenna.
 26. The power supply apparatus in claim 24further comprising a second P+ contact located in said substrate andoperative to provide power to an antenna.
 27. A method for supplyingpower to a photo-activated transponder, said method comprising the actsof: generating a positive voltage with a first photovoltaic element;generating a negative voltage with a second photovoltaic element;supplying power to a load with both said positive voltage and saidnegative voltage.
 28. The method of claim 27 wherein the act ofsupplying power to a load with both said positive voltage and saidnegative voltage comprises supplying a voltage differential between twonodes, the voltage differential being greater than the individualpositive and negative voltages.